Mauro Madia

A tool for the structural integrity assessment of turbine disks: probabilistic and numerical background

Rotor disks for gas turbines are heavy components and are usually designed following a safe-life approach, where the low-cycle fatigue analysis is carried out referring to design life curves with suitable probabilistic margins. However, in the case of such a heavy component there is the possibility of rare occurrence of undetected defects or, better, the need to identify the defect acceptability for the different rotor regions (considering stress, temperature, mission profile).In order to carry out such calculations for a turbine rotor disk a software named AStrID (Assessment of the Structural Integrity of Disks) has been developed in close cooperation between Politecnico di Milano and Ansaldo Energia (AEN). This paper summarizes its background and some of the relevant features.© 2013 ASME

Multiaxial fatigue and defect assessment of truck stabilisers

Automotive stabilisers can be considered safety components and it is, therefore, important to assess their fatigue behaviour in terms of real load conditions as well as material strength. For the second point, a detrimental effect in the fatigue limit of high strength steels is given by the presence of defects in the component. For this reason, in this paper, starting from FEM analysis in order to obtain the stresses in the component, a multiaxial fatigue assessment taking into account defects is carried out considering round and planar surface defects in terms of their SIFs. Results are then analysed by comparing maximum SIF against the threshold for short cracks, which was obtained experimentally for a quenched and tempered microalloyed steel, which represents a standard option for this kind of component. Critical areas are then identified, in agreement with fatigue full scale tests and some applications of the method for design optimisation are presented.

Analysis of Cu-wire pull and shear test failure modes under ageing cycles and finite element modelling of Si-crack propagation

Abstract In microelectronic packaging, wire bonding is the predominant method for making electrical connections. Copper is increasingly substituting gold as interconnection material since it is a much cheaper alternative and it also offers several physical advantages. Adequate and reliable mechanical integrity of the connection is usually checked by process controls based onto “wire pull” and “ball bond shear” tests. In this paper the two methods are compared in terms of sensitiveness in detecting a latent weakness of the bond-pad structure, either induced by inappropriate wire bonding process or cumulated during reliability ageing. The failure modes (in terms of frequency and maximum test load) observed at the ball bond interface have been investigated on two different batches of a same chip, obtained from different wire-bonding recipes and including both unstressed and aged units. Cross-sections of the samples, submitted to pull and shear both in destructive and non-destructive tests, have allowed us to investigate the relationship between the bond morphological characteristics (metal deformation and potential micro-damages induced by copper bonding) and the weak points for fracture propagation inside the bond-pad inner layers and the silicon substrate. Besides the experimental activities, fracture mechanics and the finite element method have been employed to model the pull and shear tests. The aims of the finite element modelling have been to predict the reduction of test maximum load in defective ball bonds and the crack growth angle adopting a mixed-mode criterion. Good results have been obtained by the numerical fracture analysis, which can then support the reliability characterization and mechanical improvement of the bond.

Critical Speed of Flawed Rotors: Global vs Local Approach

The burst of a disc in rotating machinery can cause catastrophic damage of the equipment and, more importantly, it can represent a mortal threat to anyone in the sphere of influence of the event. In order to minimize the danger associated to a rotating component failure, burst testing is required by the authorities in order to set safety margins to the normal operating speeds. Moreover more accurate predicting tools are required for designing the components. This paper presents the results of a numerical and analytical study on the assessment of the crack driving force for discs containing surface flaws. The aim is to provide a simple, though reliable, tool in order to be able to calculate the possible dangerous in-service speed for a rotating component and, therefore, to set burst margins above the normal operating conditions of the equipment. An R6-like flaw assessment procedure is adopted considering different assumptions in the definition of the critical speed and the analytical predictions are compared with the results of elastic-plastic finite element analysis of disc. In particular, the validity and potentiality of the method is proved for surface flaws in different positions in the disc.Copyright